Thermoplastic film structures are used in a wide variety of applications. One type of film structure commonly used in the art is a biaxially oriented polypropylene (BOPP) film structure, which comprises at least one polypropylene-containing layer. BOPP film is widely used to laminate to substrates such as paper or paper board for applications including book covers, food packaging, toy packaging or cardboard boxes. BOPP film not only affords water resistance but also provides a high gloss surface, which is desired in many applications. However, BOPP film also exhibits some less than desirable traits for some applications.
For example, in lamination applications, the film structure must achieve strong lamination bonds with the substrate to which it is being laminated. Traditional BOPP films do not readily adhere to substrates such as paper or paper board. Typically, this deficiency was overcome by adhesive lamination which uses a solvent or water based adhesive on BOPP film surface (sometimes in combination with treating the BOPP film surface, such as by corona discharge treatment, flame treatment, plasma treatment or metallizing treatment) and then bonds the oriented film with the substrate. The water or solvent is then removed using a long heating oven which is very energy consumptive. This method also generates pollution, and possible odor/taint problems in the final laminate. The efficiency of this process is also low due to the low speed of the drying process, and the requirement of an additional step to apply the adhesive.
Other popular approaches in the industry include extrusion coating an ethylene vinyl acetate (EVA) layer onto a BOPP surface. This process adds expense due to high cost of EVA and also adds complexity due to the additional extrusion coating process.
Other attempts and improving the bond strength of the BOPP layer to the substrate include multilayer structures where the BOPP layer is bonded to a sealant layer which in turn is bonded to the substrate. In U.S. Pat. No. 6,844,079, for example a sealant layer comprising a carboxylic acid or an ester of a carboxylic acid is taught. Such carboxylic acid containing materials, however, do not adequately bond the BOPP layer, thus requiring intermediate tie layers. This again adds complexity and expense to the film design. Accordingly, it would be beneficial for some applications if polymers comprising units derived from carboxylic acids or esters of carboxylic acids could be eliminated or at least greatly reduced.
US 2004/0105994 teaches a multilayer film with a base layer containing a thermoplastic polymer and at least one outer layer containing a low melting point polymer. This reference favors an outer layer comprising a carboxylic acid derived polymer and the inclusion of intermediate layers to promote the adhesion between the preferred outer layer and base layer. This also adds complexity and expense to the film design.
Accordingly, there is a market need to develop a solvent and adhesive free lamination process, which does not require additional coating processes or intermediate tie layers.
The present invention meets at least some of these objectives by providing in one aspect of the present invention, a film structure is provided which comprises a first layer comprising homopolymer polypropylene, and a second layer comprising a polyolefin material with a melting point of from 90° C. to 105° C. The second layer in such film structure is at least three microns thick.
In another aspect of the present inventing, a laminate comprising a substrate and a film structure laminated thereto is provided, in which the film structure comprises a first layer comprising homopolymer polypropylene, and a second layer comprising a polyolefin material with a melting point of from 90° C. to 105° C. The second layer in such film structure is at least three microns thick.